1. Field of the Invention
This invention relates to a mold device for fabricating a disc substrate constituting a disc, such as an optical disc or a magneto-optical disc, as a recording medium for information signals, by molding synthetic resin, and a disc substrate fabricated using the mold device.
2. Description of the Related Art
There has hitherto been proposed an optical disc comprising a disc substrate fabricated from synthetic resin exhibiting light transmitting properties, such as polycarbonate resin. As the disc of this type, there are currently known a read-only optical disc for only reproducing recorded information signals, and a magneto-optical disc capable of overwriting the recorded information signals. The read-only optical disc comprises a disc substrate having a predetermined pattern of pits and lands corresponding to information signals, such as music signals, formed thereon, and a reflective film formed by e.g. Al evaporation on one of the major surfaces of the disc substrate. On the other hand, the magneto-optical disc capable of overwriting information signals comprises a disc substrate having a signal recording layer including a magnetic film formed on one of its major surfaces bearing a number of pre-grooves defining a recording tracks for recording desired information signals.
It is possible with this type of the optical disc to record information signals, such as digitized music signals, with an extremely high recording density. The present Applicant has already proposed an optical disc which is 64 mm in diameter and capable of recording/reproducing or only reproducing music signals continuously for at least 74 minutes.
This optical disc 1 has a disc substrate 2, which is fabricated by molding synthetic resin exhibiting light-transmitting properties, such as polycarbonate resin, and on one major surface of which a perpendicular magnetic recording film, for example, is deposited for forming a signal recording layer, as shown in FIG. 1. For recording information signals on the optical disc 1, a light beam emitted from an optical pickup is radiated on the recording track formed in a signal recording section provided with the signal recording layer, whilst the optical disc 1 is rotationally driven at a preset rotational velocity and an external magnetic field modulated in accordance with information signals to be recorded is supplied thereto from an external magnetic field generating device, not shown.
Since the optical disc 1 is of a reduced diameter and has fine recording tracks formed thereon with extremely high density, it is necessary for the disc to be loaded in position on and made fast with a disc table 4 of a disc rotating driving unit 3 provided within a disc rotating/reproducing apparatus adapted for rotationally driving the optical disc 1, so that the disc may be rotationally driven in a correctly timed relation with respect to the rotation of the disc table 4.
For this reason, there is formed, on a major surface 2c opposite to the surface 2b of a main portion 2a bearing the signal recording layer of the disc substrate 2 constituting the optical disc 1, a central raised portion 8, the distal end face of which has a loading reference surface 7 for accurately setting the loading height position of the disc on the disc table 4. The raised portion 8 is formed as an annulus having an engaging aperture 6 adapted for being engaged by a centering member 5 which is provided at a center of rotation of the disc table 4 and which is adapted for assuring correct centering of the optical disc 1 loaded on the disc table 4. The engaging aperture 6 is adapted for communication with a centering aperture 6a provided in the main portion of the substrate 2a.
A thin metal plate 9 is provided on one of the major surfaces 2b of the disc substrate 2 for closing the centering aperture 6a and is adapted for being magnetically attracted by a magnet 10 provided on the disc table 4 for unifying the optical disc 1 loaded on the disc table 4 to the disc table 4.
The above-described optical disc 1 is loaded on the disc table 4 at the correctly set loading height position, with its center coincident with the center of rotation of the disc table 4, by having the centering member 5 engaged in the engaging aperture 6 and by having the loading reference surface 7 on the end face of the raised portion 8 supported on a disc supporting surface 4a of the disc table 4, with the metal plate 9 being then attracted by the magnet 10, as shown in FIG. 2.
The disc substrate 2 of the optical disc 1 is fabricated by injection molding of synthetic resin, such as polycarbonate, by an injection molding metal mold device.
A metal mold device as shown in FIG. 3 has been proposed for molding the disc substrate 2. The metal mold device includes a metal mold unit 1S made up of a fixed metal mold 11 and a movable metal mold 12 which is provided facing the fixed metal mold 11 and movably supported by a hydraulic device, not shown, for being moved towards and away from the fixed metal mold 11. A mold cavity 14 corresponding in contour to the disc substrate 2 to be molded is defined between the fixed metal mold 11 and the movable metal mold 12 making up the metal mold unit
The fixed metal mold 11 is provided with a sprue bushing 15, at the center of the mold cavity 14, for permitting the synthetic resin, such as molten polycarbonate resin, supplied from an injection molding machine, not shown, to flow into the mold cavity 4. The sprue bushing 15 has a central resin injection opening 16, via which the molten synthetic resin, supplied from the injection molding machine, is injected and charged into the mold cavity 14.
The fixed metal mold 11 has a molding surface 11a which defines the mold cavity 14 and which is formed as a smooth mirror surface to permit molding of the opposite major surface 2c of the disc substrate 2 which is to be a light beam incident surface. On the outer peripheral surface of the sprue bushing 15, there is provided an insert 18 for defining a recess 17 for molding the raised portion 18 which is to be raised on the major surface 2c of the disc substrate 2 and which has the loading reference surface 7.
On a molding surface 12a defining the mold cavity 14 of the movable mold cavity 12 is mounted a stamper 19 for forming a pattern of micro-sized pits and lands corresponding to information signals recorded on the optical disc 1 or pre-grooves defining recording track(s) formed on the signal recording section of the optical disc 1. The stamper 19 is loaded on the molding surface 12 by having a fitting projection 21 on the distal end of a stamper holder 20 provided at the center of the movable metal mold 12 fitted into its center opening 22 and by having its outer periphery supported by a pawl 23a of an outer peripheral stamper holder
Meanwhile, the pawl 23a of the stamper holder 29 holding the stamper 19 has its inner peripheral surface formed as a molding surface for producing the outer periphery of the disc substrate 2 molded in the mold cavity 14.
On an inner periphery of the stamper holder 20, there is provided a perforating punch 24 for punching the engaging opening 6 at the center of the raised portion 8 of the disc substrate The perforating punch 24 is passed through a sleeve 25 provided on an inner periphery of the stamper holder 20 for reciprocation therein. An air passage 26 is defined between the sleeve 25 and the stamper holder 26 for releasing the as-molded disc substrate 2 from the movable metal mold 12.
For molding the disc substrate 2, using the above-described metal mold device, the movable metal mold 12 is caused to approach the fixed metal mold 11 for establishing a mold clamping state. In this mold clamping state, molten synthetic resin, such as molten polycarbonate resin, supplied from an injection molding machine, not shown, is injected and charged into the mold cavity 14 via resin injection opening 16 in the sprue bushing 15. The movable metal mold 12 is moved towards the fixed metal mold 11 for compressing the synthetic resin charged into the mold cavity 14, by way of mold clamping. The synthetic resin thus compressed is then allowed to cool for molding the disc substrate 2 corresponding in contour to the mold cavity 14. The movable metal mold 12 is then moved away from the fixed metal mold 11, by way of mold opening, at the same time that releasing air is injected via air passage 26 into the mold cavity 14 for releasing the disc substrate 2 from the movable metal mold 12. The disc substrate 1S is then taken out by a suitable take-out device for completing the molding of the disc substrate 2.
Meanwhile, with the above-described metal mold device, the stamper 17 for forming pits and lands or pre-grooves on one of the major surfaces of the disc substrate 2 is supported on the molding surface of the movable metal mold 12 by having its peripheral side supported by the stamper holder.
In an alternative metal mold device, the stamper is loaded on the disc molding surface of the fixed metal mold or the movable metal mold by having the rim of its center opening and its outer periphery supported by an inner peripheral stamper holder and by an outer peripheral stamper holder, respectively.
If the stamper is supported in this manner by the stamper holder, part of the synthetic resin is intruded into a space between the stamper and the stamper holder to produce burrs on the disc substrate being molded when the molten synthetic resin is charged into the mold cavity. If such burrs are produced on the outer peripheral stamper holder or its neighborhood, mold releasing resistance is increased excessively to render it impossible to release the as-molded disc substrate from the stamper. On the other hand, if the releasing resistance is increased excessively to render it impossible to release the as-molded disc substrate from the stamper, the as-molded disc substrate undergoes molding distortion and abnormal polarization to render it impossible to employ it as the disc substrate for the optical disc.
On the other hand, if the perforating punch 24 for punching the engaging opening 6 is provided at the movable metal mold 12 fitted with the stamper 19 for fabricating the disc substrate 2, as in the case of the metal mold device as shown in FIG. 3, a punching burr 27 tends to be produced on the rim of the engaging opening 6 directed to the loading reference surface 7 at the distal end face of the perforating punch 24, as shown in FIG. 4.
The rim of the engaging opening 6 in the disc substrate 2 engaged by the loading reference surface 7 is engaged with the outer peripheral surface of the centering member 5 for centering the disc, while the loading reference surface 7 is to be the reference surface for setting the disc loading height on the disc table 4. Consequently, with the optical disc 1, fabricated from the disc substrate 2 having the burr 27 from punching on the rim of the engaging opening 6 directed towards the loading reference plane 7, the centering member 5 cannot be engaged accurately in the engaging opening 6, so that the optical disc 1 is loaded on the disc table 64 without centering or setting the loading position with respect to the disc table 4. The optical disc loaded under these conditions is in an offset state and is rotationally driven with the disc surface deviation from horizontal to render it impossible to record/reproduce information signals accurately.
For preventing punching burrs 27 from being produced in the rim of the engaging opening 6 directed towards the loading reference plane 7 at the end face of the raised portion 8 of the disc substrate 2, there is also proposed a metal mold device for molding a disc substrate, which is constructed and arranged as shown in FIG. 5.
In contradistinction from the above-described metal mold device, this metal mold device is provided with the stamper 17 on the side fixed metal mold 11, and the perforating punch 24 for perforating the engaging opening 6a is provided on the side movable metal mold 12, as shown in FIG. 5, so that the engaging opening 6 is punched from the loading reference surface 7 to prevent punching burrs from being produced on the rim of the engaging opening 6 directed towards the loading reference surface 7.
Meanwhile, with the metal mold device shown in FIG. 5, the perforating punch 24 is provided for reciprocating movement in a sleeve 28 mounted at the center of the movable metal mold 12 for defining the recess 17 corresponding in contour to the raised portion of the disc. The sleeve 28 may be reciprocated for releasing the as-molded disc substrate 2 out of the movable metal mold 12.
However, with the metal mold device, shown in FIG. 5, when the molten synthetic resin is charged into the mold cavity 14, part of the synthetic material is intruded into the space between the stamper 19 and the outer peripheral stamper holder 23 supporting the stamper 19 on the fixed metal mold 11 for producing a burr 29 on the outer rim of the as-molded disc substrate. If such burr 29, which is clamped between the stamper 19 and the outer peripheral stamper holder 23, is produced, the releasing resistance offered from the fixed metal mold 11 mounting the stamper 19 is increased excessively to render it impossible to release the as-molded disc substrate 2 from the stamper 19 reliably.
If the release resistance is increased in this manner to render it difficult to release the as-molded disc substrate 2 from the stamper 19 loaded on the fixed metal mold 11, it becomes difficult to open the mold device to take out the as-molded disc substrate from the molding metal mold 13.
Meanwhile, intrusion of the synthetic resin material into a space between the stamper la and the outer peripheral stamper holder 23 to produce burrs 29 on the outer peripheral side of the disc substrate 2 to render releasing of the disc substrate 2 from the stamper 19 also occurs in the mold device shown in FIG. 3.
Although it may be contemplated to inject releasing compressed air between the disc substrate 2 and the stamper 19 for preventing releasing failure of the disc substrate 2 from stamper 19, it is extremely difficult to inject the compressed air uniformly onto the major surface of the as-molded disc substrate 2, so that mold releasing cannot be achieved reliably.